A tunnel FET (tFET) is a transistor that obtains an on-state current by a band-to-band tunneling current, and has an advantage of exhibiting a smaller swing than that of a conventional FET. However, since the on-state current of the tunnel FET is smaller than that of the conventional FET, many proposals have been made to increase the on-state current of the tunnel FET. For example, a structure of the tunnel FET is proposed to include a source region of a first conductivity type, a drain region of a second conductivity type, and an impurity semiconductor layer of the second conductivity type (called pocket region) between an upper surface of the source region and a lower surface of a gate insulator. In this case, as the change in impurity concentration at a p-n junction plane between the source region and the pocket region is abrupter, the on-state current of the tunnel FET is larger. However, when forming this tunnel FET, there is a problem that impurities in the source region and the pocket region are diffused in performing activation annealing performed after ion implantation into these regions. As a result, the abruptness of the change in impurity concentration is lost. This makes it impossible to obtain a sufficient on-state current of the tunnel FET.